The present invention relates to image processing systems and methods having a capability for processing halftone images to retrieve continuous tone images and more particularly to systems and methods for converting halftone images without screen structure and other halftone images to continuous tone images.
Continuous tone images are converted to halftone or binary images to enable the production of printed copies. In binary form, the image has pixels which are either black or white and which are printed by applying ink or not applying ink.
It is usually difficult to process a halftone image such as for scaling and enhancement. Often, moire or distortion is introduced. Normally, therefore, the halftone image is first reconverted to a continuous tone image to enable processing and thereafter reconverted to a halftone image for printing.
Image processing systems used with printers in reprographic systems typically require a capability for converting halftone images to continuous tone images to meet reconversion needs and for converting scanned halftone images to continuous tone images that can then be processed by any of a large variety of enhancement algorithms commonly available for continuous tone images.
The halftoning process loses some image information in the conversion of the original continuous tone image to a halftone image. The reconversion of a halftone image to a continuous tone image accordingly is essentially an estimation process since the halftoning process cannot be reversed exactly to reproduce a continuous tone image identical to the original image.
One common process for converting continuous tone images to halftone images is a process called ordered dithering. The majority of images currently processed in the printing industry are dithered images since most printers can only print dithered images. Generally, ordered dithering is a process in which a scanned continuous signal from a continuous tone image is converted to a series of black (1 or ink) or white (0 or no ink) pixels with the pixel values determined by the pattern of a threshold or dither matrix to which the scanned signal is applied.
Another process used to convert continuous tone images to halftone images is called error diffusion. No special thresholding matrix is used in the error diffusion process. Instead, a single threshold is applied to the whole image. Generally, image pixels are processed sequentially, i.e., the first pixel is made either 1 or 0 according to whether its gray level is above or below a predetermined threshold value such as 0.5. The first pixel error is then carried forward and added to the gray value of the unprocessed surrounding pixels in determining whether these pixels are above or below the threshold value. The resultant errors are then carried forward, and the process is continued until the image is completely processed. Error diffused images normally can only be printed by very high resolution printers.
The classic prior art method for converting halftone images to continuous tone images, i.e., for "unscreening" continuous tone images from halftone images, applies a low-pass filter to the halftone image data. The low-pass filter method by its nature typically blurs image edges or at least loses fidelity of edge information (fine detail) as a result of the filter conversion process.
U.S. Pat. No. 4,630,125 to Roetling, and assigned to the present assignee, discloses a method of reconstructing a continuous tone image for grayscale values that have been converted to a halftone image of black and white spots. The reconstruction method involves isolation of each spot of a halftone image along with a neighborhood of surrounding spots, and, for each neighborhood, comparing a maximum screen pattern value producing a white spot with a minimum screen value producing a black spot.
If the minimum screen value giving a black spot is greater than the maximum screen value giving a white spot, then the grayscale pixel value of the isolated spot is the average of the maximum and minimum screen values just described. If the minimum screen value giving a black spot is less than the maximum screen value giving a white spot, then the process is repeated after deleting that portion of the neighborhood of surrounding spots containing the maximum or minimum screen value furtherest from the isolated spot. Use of the Roetling scheme is limited to orthographic or digitally created and stored dithered images since it is based on the regularity of dots in a half-tone image created with a dither.
Another U.S. Pat. No. 4,841,377 issued to Hiratsuka et al. discloses a method for estimating an original continuous tone image from a stored binary image. The method involves, inter alia, setting a plurality of scanning apertures in a binary image formed of a dither matrix, selecting one scanning aperture satisfying a predetermined condition for each picture element of a continuous image to be estimated, and estimating the continuous image on the basis of the number of white or black picture elements in the scanning aperture selected. The Hiratsuka method is similarly limited to dithered halftone images.
More recently, U.S. Pat. No. 5,027,078, issued to the present inventor, Z, Fan, discloses a method for converting halftone images to continuous tone images. The Fan method is an improvement over the Roetling method through the application of "logic filtering." This logic-filter method provides best results for digitally created and stored halftone images but it is also limited to dithered halftone images.
Additional prior art that has limited relevance to the present invention follows:
1. U.S. Pat. No. 4,691,366, "Image Enhancement", dated Sep. 1, 1987, filed by Paul Fenster, et al. PA0 2. U.S. Pat. No. 4,782,399, "Image Processing Apparatus With High- and Low-Resolution Image Sensors and An Edge Detector", dated Nov. 1, 1988, filed by Hiroaki Sato. PA0 3. U.S. Pat. No. 4,503,461, "Multiple Measurement Noise Reducing System Using Space-Variant Filters", dated Mar. 5, 1985, filed by Dwight G. Nishimura. PA0 4. U.S. Pat. No. 4,811,239, "Digital Facsimile/Image Producing Apparatus", dated Mar. 7, 1989, filed by Sherman H. M. Tsao. PA0 5. U.S. Pat. No. 4,841,374, "Image Processing Method", dated Jun. 20, 1989, filed by Matahira Kotani, et al.
In summary of the prior art, it has generally had shortcomings in preserving edge smoothing and avoiding edge blurring in the "unscreening" of continuous tone images from halftone images. Further, the prior art generally has had no effective capability for converting halftone images created by error diffusion to continuous tone images.